Generally, water treatment plants utilized for the production of drinking water are designed as either groundwater or surface water facilities.
Groundwater facilities, as the name would imply, are those systems that use groundwater as their source of supply (i.e., wells). Prior to designing groundwater treatment plants, extensive laboratory analyses are conducted to determine what, if any, chemical contaminants are present in the groundwater, and if removal of any of these contaminants is necessary for the water to be safe for public use, meet regulatory standards, and be aesthetically acceptable. Since most groundwater systems are inherently protected by the filtering ability of the surrounding soil, water quality characteristics remain relatively constant and treatment plant construction and process design are based upon the initial laboratory analyses. Although many of these facilities may utilize some of the same treatment processes employed by surface water systems, site specific conditions may require the incorporation of select chemical treatment processes.
By contrast, surface water facilities utilize some form of surface water as their source of supply (i.e., lakes, rivers, streams, reservoirs, etc.) and are generally much less specific in design. Due to the vulnerability of these systems to source water quality variations from accidental chemical spills, upstream runoff, and diverse seasonal conditions, surface water facilities are designed and constructed in a more generic fashion that provides for the removal of a wide variety of potential contaminants. In general, surface water facilities incorporate the same basic principles for water treatment processing, and utilize the same or similar chemical and physical methods for enhanced contaminant removal. Additional chemicals, with permanent or portable chemical feed equipment, may also be provided for the removal of specific contaminants that have historically appeared in the source water on an occasional basis.
Most surface water facilities employ the use of screens to prevent large debris (i.e., branches, aquatic life, etc.) from entering the treatment plant and interfering with equipment and/or processing. Low service pumps are used for transporting the screened source water to the treatment facility. Various chemicals may be added to the source water supply for the purpose of pH adjustment, disinfection, organic matter oxidation, and/or as an aid in the coagulation and removal of solid particles in the water. Mixing chambers called flocculators provide adequate solid-to-solid and solid-to-coagulant contact to create larger, more readily settleable solids that are removed in downstream primary (and possibly secondary) sedimentation basins. The majority of remaining suspended particles are removed by filters which consist of various types of fine aggregate materials (i.e., sand, stone, and/or carbon). Filtered water is stored in basins called clearwells and may be treated with additional chemicals to insure residual disinfectant concentrations in the distribution system. This finished water is then distributed to the customers via high service pumps and a network of distribution piping.
As previously mentioned, one or more oxidizing chemicals may be added to the source water in order to chemically alter or decrease (thru oxidation) the amount of organic matter present in the source water. This oxidation/alteration prevents the organic matter from reacting with other chemicals (i.e., chlorine) added further downstream in the treatment plant and resulting in the production of presently regulated health concern chemicals. One chemical used by some water treatment plants to accomplish this oxidation/alteration process is chlorine dioxide (ClO.sub.2).
Initially used as a means of controlling various taste and odor related problems and finished water iron and manganese concentrations, water industry interest in chlorine dioxide as an alternative oxidant/disinfectant increased significantly with the promulgation of the 0.10 mg/L maximum contaminant level (MCL) for total trihalomethanes (TTHMs) in 1979. After being recommended by the U.S. Environmental Protection Agency (USEPA) as a suitable technology for controlling TTHMs in 1983, the use of chlorine dioxide in U.S. water treatment plants was estimated to have tripled by 1986.
While capable of reducing the concentration of TTHMS, the use of chlorine dioxide inherently results in finished water residual concentrations of chlorine dioxide, chlorite ion (ClO.sub.2.sup.-) and chlorate ion (ClO.sub.3.sup.-), which are, themselves, targeted for future regulatory consideration by the USEPA.
In the article entitled "Using Reducing Agents to Eliminate Chlorine Dioxide and Chlorite Ion Residuals in Drinking Water", American Water Works Association Journal, May 1991, pg. 56-61, there is discussion regarding the possible use of ferrous chloride for the removal of residual chlorine dioxide and related by-products from water. Specifically, there is a discussion of performing jar tests with ferrous chloride for the removal of chlorine dioxide and the by-product chlorite ion. There is not, however, any discussion regarding the effective integration of ferrous chloride into a drinking water treatment facility for this purpose.